Magnetic resonance (MR) imaging uses the nuclear magnetic resonance phenomenon to produce images of internal patient volumes. MR imaging generally involves subjecting tissue to a uniform main magnetic field. This causes the individual magnetic moments of the nuclear spins in the tissue to process about the magnetic field at their characteristic Larmor frequency as they attempt to align with the field, and produces a net magnetic moment Mz in the direction of the main magnetic field. The tissue is then subjected to a radiofrequency (RF) field near the Larmor frequency, which may rotate, or “tip”, the net magnetic moment Mz into the x-y plane at a corresponding flip angle to produce a net transverse magnetic moment Mt which is rotating, or spinning, in the x-y plane at the Larmor frequency. Next, the RF field is terminated, causing the excited spins to emit signals as they return to their prior state.
Magnetic field gradients Gx, Gy, and Gz are used to distort the main magnetic field in a predictable way so that the Larmor frequency of nuclei within the main magnetic field varies as a function of position. Accordingly, an RF field which is near a particular Larmor frequency will tip the net aligned moment Mz of those nuclei located at positions in the distorted magnetic field which correspond to the particular Larmor frequency, and signals will be emitted only by those nuclei after the RF field is terminated. The emitted signals are detected, digitized and processed to reconstruct an image using one of many well-known MR reconstruction techniques.
An MR sequence describes a set of timings and characteristics of the above-described fields, gradients and pulses, which may be used to acquire an MR image. Known steady-state free precession MR sequences maintain a steady-state of longitudinal magnetization and transverse magnetization by applying balanced magnetic field gradients between successive equidistant RF pulses. The term “balanced” refers to zero net gradient-induced phasing over a TR (i.e., repetition time) interval between RF pulses.
Short TR intervals are desired because, if the TR interval is too long, the spins will pick up an increasing amount of phase errors from one TR interval to the next due to off-resonance effects. MR imaging systems are currently capable of applying appropriate balanced magnetic field gradients within short TR intervals, but typically at field strengths of 1.5 T and lower. At higher field strengths, increasing off-resonance effects require ever-decreasing TR intervals. Even if an MR imaging system could successfully generate and apply balanced magnetic field gradients during these shorter TR intervals, the shorter TR intervals reduce the upper limit of the flip angle. The combination of shorter TR interval and lower flip angle leads to decreased T2 weighting, which is undesirable for many types of MR imaging.